It's a nightmare,' Victoria Stapleton shouts, raising her hands high in despair. The founder and creative director of the British cashmere label Brora has waged war against the moths that have recently taken up residence in her wardrobe. 'For someone like me to have moths in my life is a real disaster. Everything has been washed and put in the deep freeze, then into sealed bags, but every night when I go into my room, I still kill a moth. They absolutely adore cashmere. They really, really do, and they eat away and they're buggers. We've never had them at work; that would be an explosion. Can you imagine?'

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It is a sunny day in July and Stapleton, 46, is sitting at a long oak table in her airy powder-blue kitchen in Hertfordshire. She is wearing a vintage Dries Van Noten floral skirt, an Isabel Marant T-shirt, a Brora belt and an amethyst ring, a present from her mother-in-law. The decor is bohemian, with reconditioned vintage furniture, a giant Aga and artwork by Emma Tennant (the mother of the model Stella and the Duke of Devonshire's sister) on the wall. Stapleton has lived here with her husband, Johnnie Pilkington (who does all Brora's photography), their three daughters, Jesse, 15, Nancy, 13, and Lola, 11, and three whippets, Honeybee, Flee Fly and Decca, since they took the house over from Pilkington's parents 10 years ago.

Moths aside, Stapleton is in a very good mood. This year Brora celebrates its 20th birthday. 'It has been a wonderfully steady journey,' she says. 'I have chosen the organic growth route rather than the aggressive "borrow loads, grow fast" route, so it hasn't been stressful. In some ways the birthday makes me feel a little old, but then I started the business quite young. On reflection what I am feeling most is pride in the company I run, which is very personal and human, the wealth of suppliers Brora has supported in the United Kingdom and the clothes we design, which seem to bring real pleasure to lots of people.'

Stapleton grew up near Penrith in Cumbria, the third of four girls. Her father, David, owned Pinneys, a Scottish smoked-salmon company, where she helped as a teenager. At 15 she spent her first pay cheque of £50 on a round-neck navy cashmere jumper (only later did she realise the significance of this). At 22 she started her first mail-order company, Pyjamarama, with her friend Georgie Channon. But after two years of just breaking even, the pair called it a day (Channon still jokes that she backed the wrong horse). In 1990 Stapleton's father invested in a 100-year-old tweed mill in Scotland, Hunters of Brora, and encouraged his daughter to save the ailing retail side of the business. The mill supplied tweed to Holland & Holland and Hackett, but the shop in the village of Brora, which also sold the tweed, was in dire straits. The potential footfall was promising because of nearby fishing and stalking estates, and her father felt that with his help she could give it the TLC it needed. She learnt about everything from product to sales: sourcing suppliers, retailing, shop layout, ordering stock, employing sales agents, marketing and running show stands. But 18 months in, the economic downturn claimed the mill. By then Stapleton felt she had learnt enough to start her own cashmere and tweed mail-order company.

Her first collection comprised some designs from her cashmere supplier's own range, some leftover cashmere stock from Hunters, a jacket in Hunters tweed made by a London tailor to her design and some shooting socks hand-knitted all over Scotland. Stapleton got the blessing of the mill to call her company Brora, inherited the mailing list of 5,000 names, moved to London, diverted telephone calls from Hunters to her flat and started trading on March 1 1993.

That same day the style writer Lucia van der Post, a friend of Stapleton's father, included Brora in a cashmere shopping article for the Financial Times. 'Those were the days when there was no internet, so people read things in the newspaper and acted on it immediately,' Stapleton says. 'The article just said, "Brora does wonderful crew necks and round necks in black, blue and brown, for £99," without a picture or anything. That first weekend of trading I took orders for £10,000 of cashmere. I hadn't even printed a catalogue at that stage and I only had one of those clunk-click card machines.'

Off to a flying start, Brora was instantly self-financing because Stapleton reinvested all profits back into the business. She did all her own book-keeping and monthly accounts using basic software on a 'huge great computer'. In July 1993 she rented a showroom-cum-office near her flat in Parsons Green, west London, printed a brochure with friends modelling the clothes, and sent it to everyone on the mailing list. A second lucky break came in September 1993 when Brora was included in a 'tried and tested' cashmere article in the Daily Mail. The fashion editor called her asking for two jumpers to try, so Stapleton chose them at random and packed them off in a Jiffy bag. Ten days later the article appeared, also featuring cashmere from brands such as M&S, Gap and Ralph Lauren, but in the centre of the page was the Brora jumper with 19 out of 20 for washability and wearability. That weekend Stapleton didn't stop answering the telephone. She had to commandeer the help of a friend for the second phone line. Together they took orders worth £100,000, writing 3,000 addresses on envelopes as they went, until the realisation came that they couldn't send out the envelopes before inputting the addresses into the computer.

Photo: Heritage Wave cardigan, £419

In Stapleton's first year of trading turnover was £180,000 (gold-buttoned cardigans were the bestsellers). That figure has increased every year since, plateauing over the past three years. Today Brora remains entirely privately owned, with an annual turnover of £17 million. The company employs 150 staff (gearing up to nearer 200 pre-Christmas), spread between the head office in Stevenage (where it moved from London five years ago to have more space and be nearer for Stapleton) and its 14 stores across Britain, and it continues to trade with most of its original suppliers. There is the Linton tweed mill in Cumbria, sewing factories in Leicester and London; the Liberty-print knickers are made in Manchester; and all cashmere clothing is manufactured at Johnston's of Elgin, a mill in Scotland. The only non-British items are belts from Buenos Aires and cotton pieces from Portugal.

Brora's cashmere suppliers have been importing raw materials from Mongolia since 1850. The cashmere fibres are ethically produced from the native goat of the Mongolian plateau. Sustainability of supply lines is critical, and Brora has bought its fibre from the same farms for many years. This is brought to Scotland, where the coarse 'guard hairs' are removed and only the finest (maximum thickness of 16.5 micron) and longest (minimum length of 34mm) fibres are used, ensuring a soft and downy end product. (Fibre falling short of these specifications will be weaker, causing a jumper to pile more quickly and lose its shape after only a season.) Stapleton relies on her suppliers' expertise to find the very best cashmere, while the dyeing, milling and knitting is done in Scotland.

As expected Stapleton is vehemently opposed to cheap cashmere. 'There is a big difference between Chinese and Scottish cashmere, in the quality of the fibre, the length, the finesse, how it's dyed, how it's milled. I've got jumpers in my cupboard that look the same as they did 10 years ago because the cashmere is very dense,' she says. 'A Brora jumper would probably be double the weight of something cheaper.' Cashmere is an expensive commodity at $150 per kilo, which makes three Brora jumpers.

Stapleton remains hands-on, overseeing a design team of four working across the full range of clothes and accessories for women, men, child­ren and babies. For autumn/winter the womenswear collection has a Scandinavian folk story, heavy on embroidery and velvet, with sharp accents of scarlet and icy blues. There is also the English Eccentrics story, influenced by Stapleton's visit to the David Hockney exhibition at the Royal Academy ('which completely blew me away'), using the deep purple, smoky peat, livid moss green, kingfisher turquoise and terracotta of his Yorkshire landscapes (the clothes are also made in the north of England). This season prints are more graphic than the ditsy florals of past seasons, reflecting a shift in Stapleton's personal style away from 'mumsy clothes'.

Collaborations with the Scottish designer Louise Gray in 2011 and Michael van der Ham last year benefited both sides and allowed Stapleton to think outside the box. 'They were really good for me creatively because they came at it from a different angle, a different aesthetic and a different design perspective. They push the boundaries,' she says. Both collections were a success and opened Brora up to new customers. Earlier this year Sophie Dahl designed a capsule collection for the brand, offering printed tea dresses and vintage-inspired cosy knits.

To celebrate Brora's 20th anniversary Stapleton is highlighting the importance of Brora's British manufacturing and support of local craftsmanship. 'Reaching 20 years feels really grown up,' she says. 'I was thinking of ways to celebrate and knew it had to be about making in Scotland, about the cashmere and tweeds, which are the reason I started this business and remain so much a part of the DNA of Brora.' Trawling back through every catalogue since the start, Stapleton picked out 14 'real gems', a mixture of bestsellers and most-requested pieces, to form the Heritage Collection (available from September 23). Each piece has been remade in limited editions of between 28 (the cashmere cable-knit cardi-coat) and 100 (the Fair Isle yoke cardigan in Scottish-spun Shetland wool). There are two Scottish plaid tweed kilts, a woollen goat brooch and a tweed hacking jacket with Liberty-print lining. The cashmere pieces include a blanket, a chunky polo neck in pale grey or jade, a crochet-collared cardigan in the colours Fresco or Cyclamen, and a pointelle grandad top with velvet trim in Cyclamen or Jade.

'There are still things in my first collections that I wear today,' Stapleton says. 'The colourful zigzag cashmere cardigan from the autumn 2005 collection is in the Heritage Collection because it's my all-time favourite cardigan. Whenever I wear it people always say, "I wish I had that cardigan." '

The fact that Brora marks its milestone birthday with a 'greatest hits' collection rather than launching off in a new direction demonstrates a quiet self-confidence and pride that is rare on Britain's high streets. 'We don't have to reinvent ourselves,' Stapleton says. 'We can still sell cashmere wrist-warmers every year for the next 30 years, but we've just got to keep it exciting.'